Measure all preantral follicles in 27 pairs of rhesus monkey ovaries removed during documented stages of the menstrual cycle. Antral follicles from the same ovaries have already been measured and the anatomical description of these follicles has been correlated with hormonal blood levels. The detailed analysis of the preantral follicles from the same ovaries should provide additional information on ovarian dynamics and indicate whether follicles of the same size are grouped together and what relationship they have to antral follicles and corpora lutea.